Field of the Invention and Related Art
The present invention relates to a color toner used for converting an electrostatic latent image to a visible image in an image forming process such as electrophotography or electrostatic recording.
In recent years, with wide spread of image forming apparatus such as color copying machines for electrophotography, they have come to be widely used for various purposes and also severely required to satisfy image quality. In the copying of images such as photographs, catalogs or maps in common use, it is demanded for them to be very finely and faithfully reproduced throughout their details without any crushed or broken images.
In image forming apparatus such as color copying machines for electrophotography that recently employ digital image signals, a latent image is formed as a group of dots having a given potential, and a solid area, a half-tone area and a light area are expressed by variation of dot density. There, however, is a problem that the gradation of a toner image, corresponding to the ratio of dot density at a black area to dot density at a white area of a digital image, can not be obtained when toner particles are in such a state that they do not accurately cover the dot and are protruded therefrom. Moreover, when the dot size is made small to improve the resolution so that image quality can be improved, it becomes more difficult to achieve fidelity of reproduction of a latent image formed of minute dots, tending to bring about an image having a poor resolution, in particular, a poor gradation at a highlight area, and lacking in sharpness.
It sometimes occurs that an image has a good image quality in the initial stage but turns out to have a poor image quality in the course of continual copying or printing-out. This phenomenon occurs presumably because only toner particles that have good developability are preferentially consumed in the course of continual copying or printing-out, and toner particles that have poor developability are accumulated and remain in the developing machine.
For the purpose of improving image quality, several developing agents have been hitherto proposed. Japanese Patent Application Laid-Open No. 51-3244 (corresponding to U.S. Pat. No. 3,942,979, No. 3,969,251 and No. 4,112,024) discloses a non-magnetic toner in which particle size distribution is controlled, aiming at an improvement in image quality. This toner mainly comprises a toner with a particle diameter of from 8 to 12 .mu.m, which is relatively coarse. Studies made by the present inventors have revealed that a toner with such particle diameter can not uniformly densely "cover" a latent image. In addition, it has a broad particle size distribution in view of the characteristics that particles with a diameter of 5 .mu.m or less account for not more than 30% by number and those of 20 .mu.m or more account for not more than 5% by number. This tends to lower uniformity. In order to form a sharp image by the use of such a toner containing a coarse toner particles end also having a broad particle size distribution, it is necessary to provide toner particles in a large thickness so that there can be no spaces between particles, thereby increasing apparent image density. This also brings about the problem of an increase in consumption of the toner necessary for attaining a given image density.
Japanese Patent Application Laid-Open No. 54-72054 (corresponding to U.S. Pat. No. 4,284,701) discloses a non-magnetic toner having a sharper distribution than the above toner. However, the size of particles with an intermediate weight is as coarse as from 8.5 to 11.0 .mu.m, and there is a room for further improvement for a color toner capable of faithfully reproducing minute-dot latent images and giving a high resolution.
Japanese Patent Application Laid-Open No. 58-129437 (corresponding to British Patent No. 2,114,310) discloses a non-magnetic toner having an average particle diameter of from 6 to 10 .mu.m and in which the particles present in the greatest number have a diameter of from 5 to 8 .mu.m. Since, however particles of 5 .mu.m or less account for as small as not more than 15% by number, an image lacking in sharpness tends to be formed.
As a result of studies made by the present inventors, it has been found that toner particles with a diameter of 5 .mu.m or less can definitely reproduce minute dots of latent images and have the principal function that a toner can densely cover the whole latent images. In particular, in the case of an electrostatic latent image on a photosensitive member, an edge that forms the contour of an image has a higher electric field strength than the inner part thereof because of concentration of lines of electric force, so that the sharpness of an image depends on the quality of the toner particles gathering at the periphery. Studies made by the present inventors have revealed that the amount of toner particles of 5 .mu.m or less is effective for solving the problems in the highlight gradation.
However, a problem may arise such that aggregation force of the toner itself may increase with a decrease in the particle diameter of toner particles and an increase in the toner particles of 5 .mu.m or less, so that the mixing property with a carrier or the fluidity of toner is deteriorated.
For the purpose of improving the fluidity, it has been conventionally attempted to add a fluidity improver. It, however, is difficult to balance the fluidity and charging characteristics of a toner to satisfy the flying of a toner or a high image density, unless the particle size distribution and, in particular, the amount for the presence of coarse particles in the toner particles is taken into account.
Studies made by the present inventors have revealed that use of toner particles of from 12.7 .mu.m to 16.0 .mu.m contained in an amount of from 0.1 to 5.0% by volume, when toner particles of 5 .mu.m or less are contained in a proportion of from 15 to 40% by number, can achieve stable fluidity of a toner and can be effective for solving the problems.